The above-mentioned technique is commonly known in the plastics-forming art as “blow molding”. In particular, blow molding is a commonly-employed technique for the formation of containers, such as those for food, beverages, or chemicals. Common plastic resins for blow molding include Polyethylene Terephthalate (PET), High-Density Polyethylene (HDPE), and Polypropylene (PP), though other resins with properties suitable to the process and application may be employed.
A typical feedstock for a blow molding process is a hollow thermoplastic preform of generally tubular shape created by means of injection molding. Said preform is generally closed on one end, leaving the other end open, having the appearance of a test tube or similar container. The open end of the preform is generally in a form that is substantially finished, optionally including such features as threads, spouts, etc. However, the preform may be left open and unfinished at both ends, with closure and features to be added later.
In a typical blow molding process, the preform is first pre-heated, generally by use of ovens or radiant heaters. The temperature to which the preform is heated is one above the vitreous transition point of the material. This renders the preform soft and pliable, and thus capable of flowing into the recesses of a mold. The working temperature of a blow molding process thus ranges from 55° C. to 135° C., depending on the properties of the material used.
The preform, once uniformly heated to the desired temperature, is positioned within a mold. The cavity of the mold is contoured in such a way as to delineate the exterior of the finished container. The neck at the open end of the preform generally protrudes from the top of the mold. A stretch rod is inserted into the opening of the preform, and about the opening of the preform the blow molding apparatus is positioned. During molding, the stretch rod is advanced into the preform in such a way as to press into the closed end of the preform and deform it in the direction of its longitudinal axis. Simultaneously, air under high pressure is blown into the preform, causing the preform to expand and fill the mold cavity. The container is then cooled, which is accomplished either by circulating coolant through the mold body before the container is ejected, or by injecting a cryogenic fluid such as liquid nitrogen into the container. Once it has sufficiently cooled, the container is removed from the mold, filled with a product and sealed.
The present method as described above is disadvantageous in several aspects. First, the plastic preform must be pre-heated, in order to maintain the plastic at the proper temperature for forming, a step which adds considerable cost to the process in the form of energy costs. These costs are of even greater concern in high-volume operations, wherein large quantities of preforms must be brought to working temperature very rapidly and large installations of heating equipment are required. Second, as the containers are molded using gas, control over the molding process is diminished by the compressibility of the gas. Third, the container must be cooled after molding and may not be filled until its temperature has decreased sufficiently, which limits the speed at which a blow-molding apparatus may operate and generally adds another layer of complexity and expense to the process. Fourth, as the temperature of the plastic is maintained above the material's vitreous transition point during the forming process, the molecular structure of the finished container remains largely amorphous, and does not benefit from the increases in strength and other physical properties that are the result of crystallization and strain hardening.